open-cv findChessboardCorners fails to detect corners

Question

This is the image (24 inner-corners from left to right and 15 from top to bottom):

This is the code:

img = cv2.imread('path to image')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
val = cv2.CALIB_CB_ADAPTIVE_THRESH + cv2.CALIB_CB_FAST_CHECK + cv2.CALIB_CB_NORMALIZE_IMAGE
ret, corners = cv2.findChessboardCorners(gray, (24, 15), val)

The code for another (very similar image but with more space on the left hand side) seems to work fine:

I fail to understand what the issue is (I am using python 3.8.8 with open-cv 4.5.3)?